Femto Photography

Photography has reached to an extent when capturing motion with a speed near to 300 million meters/second is no longer a limit!

Photography has become an integral part of our lives. We click photos all the time, share them, get them liked by the people. But have we ever thought how this technology developed and where are we today in the science of photography?
The science of photography has developed a lot in the recent years. Femto-photography is one of such innovations which is the latest and is the most significant of all times. Let’s talk about a technology little inferior to the Femto photography, high-speed photography. We have all seen slow motion or even super slow motion videos of bullets moving. These movies are just photographs taken at a very high speed i.e. around 1000 frames (photographs) per second (fps), which is the same principle of normal videos you watch but are at a low fps (24fps). Our human eye has a retention time of 1/16th of a second, which means if 2nd photograph passes in front of our eyes before completing 1/16th of a second, we can’t tell that those 2 photos are distinct. They will appear as continuous. This principle is used in movies which is why they are also called motion pictures.

Now imagine taking 1 trillion frames (photographs) per second! This is the extent of Femto photography. With this technology, we can even see light moving, the waves of light hitting the object and interacting with it. But before we go further, let’s take a dive into the history of photography and see how the photos looked back in 4th century BCE.

Photography history and comparison with new age photography

Photography has its roots from around 5th-4th century BCE. Ancient Han Chinese philosopher Mo Di from the Mohist School of Logic was the first to discover and develop the scientific principles of optics and pinhole camera. After that, other attempts were made by Aristotle, Anthemius of Tralles, Ibn al-Haytham, Albertus Magnus and many others to pass light through a pinhole camera and project image on paper or some light sensitive material.

This photograph, the oldest known in the world, represents a Dutch engraving showing a man leading a horse. It was made by the French inventor Nicéphore Niépce in 1825.

From several of the attempts made, the first ever known attempt to capture the image was by a British inventor Thomas Wedgwood in 1800. He used paper or white leather treated with silver nitrate. Although he succeeded in capturing the shadows of objects placed on the surface in direct sunlight, and even made shadow copies of paintings on glass, it was reported in 1802 that the images formed by means of a camera obscura have been found too faint to produce, in any moderate time, an effect upon the nitrate of silver. The shadow images eventually darkened all over.

The modern day photography as we see today uses the same principle of focusing the light emitted or reflected from the surface as the pinhole cameras. The difference is that it uses an electronic image sensor that produces an electrical charge at each pixel, which is electronically processed and stored in a digital image file for subsequent display or processing.

Image showing bullet piercing an apple

For all the advancements in technology, we have created high-speed cameras which can capture image exposures in excess of 1/1,000 or frame rates in excess of 250 frames per second(fps). The normal videos that we watch are close to 24 fps. Today we can have resolutions up to four megapixels at record rates over 1000 frames per second, which means in one second the user will have over 11 gigabytes of image data.
High-speed film cameras achieve these high frame rates by running the film over a rotating prism or mirror instead of using a shutter, thus reducing the need for stopping and starting the film behind a shutter which would tear the film stock at such speeds. Using this technique one can stretch one second to more than ten minutes of playback time!

Now imagine going even beyond this high-speed photography, seeing the light in slow motion. See the light hitting an object and bouncing back, following a path and finally to a point when you see light with your naked eyes.
Researchers at MIT have found a way to capture light by devising a camera that can capture more than trillion frames per second!

Femto Camera

Photos taken from femto camera shows an approaching light wave just before and after striking a tomato.
Let’s take a dive into what exactly happens behind femto photography.
The experiment conducted at MIT consisted of a titanium-sapphire laser source that emits pulses at regular intervals of ~13 nanoseconds, an object kept in front on which the laser is going to fall, and a ”streak tube” which captures light returning from the scene.

Before going any further, let’s know what a streak tube is?
Streak tube behaves like an oscilloscope with corresponding trigger and deflection of beams. A light pulse enters the instrument through a narrow slit along one direction. It is then deflected in the perpendicular direction so that photons that arrive first hit the detector at a different position compared to photons that arrive later. The resulting image forms a “streak” of light.

Working of femto camera

The object is placed in front of the titanium-sapphire laser. The pulses from the laser illuminate the object and also trigger the streak tube, which captures light coming from the object. The streak camera has a reasonable field of view in the horizontal direction but very narrow (roughly equivalent to one scan line) in the vertical dimension. So, this records a 1D movie of this narrow field of view. In the movie, a recording of roughly 480 frames is done and each frame has a roughly 1.71 picosecond exposure time.

Now to convert this 1D movie into a 2D movie, i.e recording in the vertical dimension as well, a set of carefully placed mirrors then orient the camera towards different parts of the object and capture a movie for each view. Finally, a powerful computer combines all the 1D movies into a single 2D movie of around 480 frames.

Judging by the process, we can say that the Femto camera and high-speed camera are same? NO! The high-speed camera records a single bullet and slows the motion down (due to its high frame capturing per second) for us to see. But Femto camera does not capture a single photon of light and slows it down. It takes millions of photons passing through a small space and assuming all follow the same path, sensors then capture the photons on streak tube and the values are averaged out to show a movie of “streaks” of light falling on the object. So what we see in the image is not how a light is travelling at a particular instant but is the average of millions of photons falling on the same object!

Apparatus

This is a clever use of science to do an impossible thing. This streak tube, laser and mirror apparatus is also currently being used to look around the corners! An object can be defined by a laser hitting the object again and again and reflected back into the streak tube. And same as weaving the light streaks together, these reflections from the object can be recorded and can depict a rough shape of the object around the corner.

Image showing the result of subsequent weaving of millions of reflections of the object around the corner.

The advent of Femto Photography is not an end but just the beginning. We are definitely going to see some better techniques to further improve this. Technology has developed further to increase frame rates to a whooping 4.4 trillion frames per second. With this rate, who knows, maybe we’ll find an actual “Tachyon” ( a hypothetical object which is faster than light!) one day.